Helicobacter pylori is a curved, microaerophilic, gram-negative bacterium that is the major cause of chronic gastritis in humans. While most infected persons remain asymptomatic, infection with this pathogen is a significant risk factor for the development of peptic ulcer disease and gastric carcinoma. The major objective of the proposed work is to characterize the role of the 131 kDa immunodominant antigen (CagA; cytotoxin-associated gene product) in the pathogenesis of H.pylori- associated duodenal ulcer disease. About 50-60% of H.pylori isolates produce the CagA antigen, and virtually all H. pylori-infected patients with duodenal ulceration develop a serologic response to CagA. Previous studies have shown that mucosal neutrophil infiltration and epithelial surface degeneration are significantly greater in patients with a local gastric immune response to CagA than in those without such a response. Previous studies have also shown that CagA-producing H. pylori strains induce significantly more interleukin-8 (lL-8) synthesis by gastric epithelial cells than do CagA-negative strains. In addition,recent studies have shown that patients infected with CagA+ strains induce a specific pattern of cytokine expression in gastric tissue involving IL-1 alpha and IL-1 beta. The gene encoding CagA has now been cloned, and recombinant CagA protein is recognized by sera from an H.pylori-infected patients. H.pylori can vary the size of CagA and the size variation is associated with deletion or insertion of repeat sequences. Sequence analysis of the region upstream from cagA, which is not present in CagA strains, identified three genes (cagB, cagC, and cagD ). The deduced CagC sequence was found to have significant homology with the Bordetella pertussis toxin secretion protein (PtIC). The hypothesis of this study is that the CagA antigen of H.pylori is associated with duodenal ulcer disease, and is an important virulence factor. The specific aims are 1) to study the relationship between CagA production and H.pylori-associated duodenal ulcer disease, 2) to determine the role of CagA in gastric inflammation, 3) to characterize the genetic differences between CagA+ and CagA H.pylori strains.To accomplish the first objective, native CagA from H.pylori strain 84-183 will be purified and its structural and biochemical characteristics determined. Next, mucosal immune responses to CagA will be compared in patients with gastritis alone and those with duodenal ulceration. Subsequently, H.pylori isolates from patients with gastritis alone and duodenal ulcer will be tested for CagA size and determine the relationship between the type of CagA expressed and clinical outcome. Isogenic pairs of CagA+ and CagA strains will be used to determine whether CagA plays a role in cytokine induction and neutrophil activation within gastric tissues. The cagA neighboring genes that are present exclusively in CagA+ strains will be characterized and disrupted by insertional mutagenesis, and the mutants will be tested for cytotoxin secretion and/or cytokine induction. Understanding the pathogenic role of CagA and its neighboring gene products in H.pylori infection may aid in the development of strategies to decrease the morbidity associated with complications of H.pylori infection.